The present invention relates to a method of producing hydrogen cyanide by the gas-phase reaction of methane with ammonia at elevated temperature and operating with an ammonia excess of at least 0.1 molar %.
Hydrogen cyanide is currently produced on an industrial scale according to the Andrussow method or by the BMA method (see Ullmann's Encyclopedia of Industrial Chemistry, volume A8, Weinheim 1987, pages 161-163). In the Andrussow method the reactants which are methane, ammonia and oxygen are reacted at temperatures above 1000.degree. C. in the presence of a catalyst to form hydrogen cyanide and water. The catalyst is typically a wire mesh made of a platinum/rhodium alloy or platinum/iridium alloy. The reaction can be represented as: EQU 2CH.sub.4 +2NH.sub.3 +30.sub.2 .fwdarw.2HCN+6H.sub.2 O
In order to obtain an optimum yield, the prior knowledge has indicated that a very short contact time (milliseconds) is required and consequently a high operating temperature is required to reach equilibrium. Typical conversion efficiencies reported in the art are of the order of 60-70%.
A representative catalyst used in this commercially known process is 10% rhodium, 90% platinum using a wire diameter of 0.076mm. Until the introduction of knitted catalyst this was traditionally woven gauze at 1024 mesh/cm.sup.2. As known in the art a 10% rhodium/platinum has been used as a typical alloy although other alternatives have also been tried.
In a typical installation depending on the reactor type and operating conditions, between 6 and 40 gauzes are installed. As is known, the catalyst can be supplied as a single pad or in pads of 3 or 4 gauzes.
According to observations in the industry, during operation the catalyst wires can expand by approximately 25% with the top gauzes becoming completely porous. The degree of porosity normally decreases with increasing thickness and at the bottom most gauzes still retain a solid core. However, during the process the gauzes may become sintered together making separation extremely difficult. As is the case in ammonia oxidation the surface of the wire develops a crystallite structure which significantly increases the available surface area.
As the majority of plants operate at atmospheric or low pressure, metal losses are minimal compared to ammonia oxidation. Typical losses are of the order of 3% of the installed weight per cycle.
Contamination of the gauze surface, particularly with iron, can have a very adverse effect on the efficiency of the reaction. Sulphur, silica and aluminum are also contaminants that will significantly reduce catalyst performance. Other common problems include gas distribution and obtaining an homogenous gas mixture. Frequent shutdowns can also cause the catalyst pack to shrink.
In the BMA method, methane and ammonia are reacted at temperatures above 1300.degree. C. on a platinum-containing catalyst (see EP 0,407,809 Bi). Hydrogen is also produced along with the hydrogen cyanide. The BMA method is carried out primarily in a so-called multitube fixed-bed reactor or multitube-flow reactor. The reaction tubes consist essentially of aluminum oxide and are provided on their inner surface with the platinum-containing, catalytically active coating. In order to maintain the reaction temperature the tubes are suspended in the interior of a combustion chamber and the combustion gases flow around them. The reaction tubes are typically approximately 2 m long and have an inside diameter of approximately 16 to 18 mm.
In order to produce the hydrogen cyanide, a mixture of ammonia and methane is conducted through the reaction tubes and heated very rapidly to approximately 1300.degree. C. at normal pressure. In order to avoid the formation of problematic deposits of carbon black on the inner surfaces, the molar ratio of ammonia to methane is maintained in a range of 1.001 to 1.08.
The BMA method achieves yields of hydrogen cyanide of approximately 90% relative to the methane used. The BMA process is endothermic. The energy used for production is approximately 40 MJ per kilogram of hydrogen cyanide produced. The amount of energy per kilogram hydrogen cyanide can be reduced approximately in half by using a monolithic countercurrent reactor as shown in DE 195 24 158.
Both the Andrussow method and the BMA method require high expenses for investment and operation. On account of the high reaction temperatures correspondingly expensive technical designs are required for the reactors. Even the platinum-containing catalysts represent a large investment expense and must be replaced after approximately 10000 to 15000 operating hours on account of increasing deactivation.
The Andrussow method and the BMA method are suitable only for the large-scale production of hydrogen cyanide. A single reactor can have an output of 5000 to 20000 tons per year. The reactors run 1 to 2 years without interruption until the catalyst must be regenerated or replaced. The starting of the reactors is time-consuming and usually takes several hours. No economical method is available for medium production amounts of up to 1000 tons per year.
An object of the present invention therefore is to produce hydrogen cyanide which operates at essentially low temperatures, and without catalysts.
Another object of the present invention is to provide a process for the production of hydrogen cyanide in rather small amounts.